How fast, Officer?

The automobile becomes instrumented

Feature Article from Hemmings Classic Car

It was only a matter of time before the automobile would become powerful enough that excessive speed would become an issue. State and local laws would eventually be implemented to limit the velocity of an automobile in certain areas. This was a concept that was unheard of with a horse-drawn carriage and was not especially embraced by a freedom- and freewill-loving American public. But before traffic laws and speeds could be legally enforced, the driver of the automobile would require a means of keeping tabs on how fast he or she was going.
Once instrumentation became standard issue in automobiles, the engineers quickly adapted the information provided by the speedometer to test their designs. The ability to quickly and instantaneously determine velocity under various conditions was a major step forward in the advancement of vehicular technology.
Thus, the speedometer can be defined as an instrument for measuring and indicating the exact speed at which a car is traveling. In the United States, the scale was miles traveled per hour. The velocity is registered with some style of needle or dial and represents the speed at the instant of indication. For this reason, the reading constantly changes as the vehicle's forward progress is altered by imperfections in the roadway, goes up or down hills, or is controlled by a driver who does not possess smooth skills.
Most speedometers, even in the earliest applications, also employed a distance counter that is called an odometer. This instrument is used to measure and record, in miles and tenths of a mile, the distance a car has traveled over its lifetime. A separate odometer identified as a trip odometer was often included and could be reset at will to count a specific distance, such as a weekend drive or the miles traveled on a tank of gasoline.
The odometer was usually encased with the speedometer and mounted in the dashboard.
Though it would be hard to pinpoint the actual first year a production car employed a speedometer, it is generally accepted that around 1920 would be a good estimate. Due to the many and often short-lived vehicle manufacturers during the early 20th century, historians can not arrive at a consensus for the inclusion of speed-registering equipment in cars.
Principles of Operation
Early American automobiles employed one of four different theories of speedometer operation. The design was either magnetic, in which a revolving magnet exerted drag on a dial; centrifugal, in which revolving weights supplied the energy to move a needle through centrifugal force; air, where a current of air flowed against a vane carrying the dial; or liquid, in which a column of liquid was lifted a height proportional to the speed of a pump drive attached to the vehicle.
Regardless of the speedometer instrument design, a drive method was required. The most common was a drive that was attached to a front wheel, with an alternative location being the driveshaft or transmission shaft. Gears were employed to transfer the motion into a cable that traveled to the speedometer head.
A transmission drive, instead of placing the speedometer drive on the wheel, which was the most common in the beginning, has the drive gears just in the rear of the gear box's main shaft. A swivel joint and gear section were usually part of the design, while some manufacturers opted for a set of gears inside the transmission case to drive the instrument.
Once the drive choice was made, the engineer needed to choose the method by which the speedometer would operate.
The magnetic principle, as employed by the Stewart-Warner Company, utilized a revolving magnet positively driven from the drive unit. The magnet exerted its influence on a metal part, which was separated from it by an air gap, which in turn was connected with the indicating mechanism. The metal part was generally aluminum, as the inertia of the part must be kept as low as possible to make the speedometer quickly sense changes to the vehicle speed. A feature of the magnetic design was that the travel of the dial was a direct ratio to the speed of travel of the magnet. In order to compensate for changes in drag due to temperature differences, a compensating unit was included.
A centrifugally controlled speedometer was very similar in operation to a fly-ball engine speed governor. Speedometer manufacturers Johns-Manville, Sears-Cross, Corbin-Brown, Hoffeker, and Garford all employed this style instrument.
The weights were mounted on the revolving shaft by bell crank levers, which allowed them to travel farther from the axis of the shaft as the speed of the drive increased. The centrifugal force of the weights increased at a rate equal to four times the speed at which the shaft turned.
The natural desire of the weights to fly from the axial center of the shaft under the influence of centrifugal force provided the basis of the indicating needle movement. An ingenious feature in centrifugal design was that although the movement of the weights would vary as the square of the speed, the levers of the cams governing the movement were so calculated that the calibrations on the dial were uniform. Another feature was the finely balanced weights. The mechanism of a speedometer of this style was so sensitive that, even at low speeds, the correct rate of travel with very minimal error was indicated.
An air principle-based speedometer calibrates an air current and translates the results into miles per hour. The air circulator consisted of two intermeshing aluminum gears housed in a chamber in which there were two openings, one from the outside and away from which the gears rotate, the other opening allowing the air into the speed dial chamber where it was directed against a light vane attached to the inside of the speed dial. This described an inverted aluminum cup mounted on a pivot set in jeweled bearings. The amount of air directed against the vane in the speed dial was governed by the speed at which the air circulator was driven by the flexible shaft (cable). The speed dial, when the car was at rest, was held to zero by a nickel steel hairspring.
The liquid or hydraulic principle employs a hydraulic system and uses a centrifugal pump which is connected with the drive and which lifts a liquid to a height proportional to the speed of the drive. The tube in which the colored liquid is lifted is calibrated to register a speed.
Ratio of Gearing
The ratio of gears for a speedometer when driven from the front wheel was determined by doubling the diameter of the tire. This provided the number of teeth necessary in the large driving or road wheel gear. For example, a 30-by-4-inch tire would require a 60-tooth gear. Driven pinions on all Stewart-Warner speedometers for front drive have the same number of teeth and drive through a 2.5:1 swivel-joint reduction. The gear reduction in the swivel-joint was mounted close to the driven pinion. When installed in the left-hand wheel, a swivel-joint was used which reversed the direction of rotation. The Stewart-Warner speedometer cable turned 1,009 revolutions per minute when the car was traveling 60 mph.
Speedometer Maintenance
As with most aspects of the early motorcar, mechanical problems, failures and routine service were just considered a way of life, and the speedometer was no exception.
If the speedometer stopped working, which was a common occurrence back then, it was necessary to check that the clutch in the flexible shaft was still attached to the neck of the speedometer and did not become disconnected.
If there was a suspicion of speedometer error, a simple test could be performed.
When the drive is from the front wheel, jack up the instrumented wheel and place a chalk mark on the rim. Then turn the wheel as fast as possible. At a signal, one person reads the speedometer while another counts the revolutions for one minute, timed with a stop watch or second hand. The number of revolutions of the wheel and the speedometer reading at the start and finish should be recorded. By adding the two values and dividing them by two, the average speed could be found.
The actual speed may be obtained by referencing a chart supplied by the manufacturer that would correlate tire size.
Dial vibration or fluttering was a common issue and often resulted in the failure of the speedometer. Noisy instrumentation could also be an indicator of a pending problem. If the speedometer was noisy, it was suggested that the following could be the cause:
Loose union connection between speedometer and flexible shaft connection.
Loose union connection between driving shaft at transmission and flexible shaft.
The flexible shaft may be bent at a sharp angle causing the shaft to bind during rotation.
The worm gear and pinion at the drive end may be either meshed in part or entirely out of mesh due to the speedometer pinion sleeve at the rear of the transmission being loose.
The flexible shaft may not be well lubricated, which causes erratic shaft movement and unsteady indication. A special grease is required to lubricate the shaft (cable).
The chain in the flexible shaft may have broken.
When the speedometer pointer vibrated, the vehicle owner was advised to look for loose unions, connections, flexible shafts bent too sharp, lack of lubrication or gears not meshing properly.
An Important Marketing Tool
As the automobile matured, and especially after WWII and the improvement of the roads and the creation of the Interstate Highway System, the speedometer became a very powerful marketing tool.
It did not take long for the advertising executives to realize that the public would hold in high esteem the maximum rating of a vehicle's speedometer, regardless of the engine that was placed under the hood. The mind set of the consumer was, "The higher the maximum scale on the speedometer, the faster and more powerful the car must be." This was based on the innocence and conventional wisdom of the time that rationalized that no company would have a speedometer graduated to 120 mph if the car could obtain a top speed of only 85 mph (which was common for a mid-1950s six-cylinder sedan). This became so entrenched in the public psyche that for the 1975 model year, the federal government mandated that all speedometers be graduated to a maximum of 85 mph. It was believed that many drivers, given the chance, would try to see if their vehicle could pin the speedometer needle. Thus, by lowering the maximum graduation, the desire for "burying the needle" did not cease, just the speed at which the bragging right would occur was lowered.
The age of electronics in the early 1980s created the digital speedometer. Much like the music and clothing of that time, it was short-lived and slipped quietly away, and Detroit returned to the analog speedometer. The only difference now is that a modern speedometer employs no cable and is operated by a vehicle speed sensor that sends a signal to an electric motor in the speedometer head. The motor then moves the indicator dial. This has eliminated all noise and the shaking needle from a cable that was bent or in need of lubrication. But even with all of the latest technology, the lowly speedometer is used as an excuse by a speeding motorists to police officers and in traffic courts around the country. It is charged with being inaccurate and the reason for the driver breaking the law. I don't think that will ever change!

This article originally appeared in the July, 2006 issue of Hemmings Classic Car.